1. Field of the Invention
This invention relates to a semiconductor laser array device which emits high-output power laser light with a single narrow beam.
2. Description of the Prior Art
Semiconductor laser devices which are useful as light sources of optical discs, laser printers, optical measuring systems, etc., must produce high output power. However, conventional semiconductor laser devices having a single waveguide structure can only produce a low output power, 60-70 mW at their best, even taking into account their window effects and/or the reflectivity control at their facets. In order to oscillate laser in a certain array mode (i.e., a 0.degree. phase-shift mode, resulting in a single narrow beam in the higher output power), semiconductor laser array devices, in which a plurality of waveguides are disposed in a parallel manner to achieve an optical phase coupling between the adjacent waveguides, have been studied. However, the optical phase shift between the adjacent waveguides of these devices is, indeed, 180.degree., and output power light is emitted into a two beam fashion having a certain angle therebetween, resulting in a far-field pattern having two peaks. Thus, this laser light cannot be concentrated into a spot fashion by means of any optical lens. FIG. 6 shows a typical conventional semiconductor laser array device 9, which can be produced as follows: On the ( 001) face of a p-GaAs substrate 1, an n.sup.+ -Al.sub.0.1 Ga.sub.0.9 As current blocking layer 2 having a thickness of 0.7 .mu.m and an n-GaAs surface-protective layer 3 having a thickness of 0.1 .mu.m are successively formed by liquid phase epitaxy. Then, three straight channels 4 are formed in a parallel manner through the surface-protective layer 3 into the p-GaAs substrate 1. Each of the channels 4 has a width D1 of 4 .mu.m and a depth D2 of about 1 .mu.m. Therefore, the distance D3 from the center of one channel to that of the adjacent channel is 5 .mu.m. These channels 4 are disposed at right angles to the (110) plane. On the surface-protective layer 3 including the channels 4, a p-Al.sub.0.42 Ga.sub.0.58 As cladding layer 5 having a thickness of 0.2 .mu.m, a p- or n- Al.sub.0.14 Ga.sub.0.86 As active layer 6 having a thickness of 0.08 .mu.m, an n-Al.sub.0.42 Ga.sub.0.58 As cladding layer 7 having a thickness of 0.8 .mu.m, and an n.sup.+ -GaAs contact layer 8 having a thickness of 1.5 .mu.m are successively formed by liquid phase epitaxy. Since the channels 4 are filled with the p-cladding layer 5, the surface of the p-cladding layer 5 becomes flat. Then, the upper face of the contact layer 8 and the back face of the substrate 1 are subjected to a vapor deposition treatment with metal materials and then heated to form ohmic contacts thereon made of alloys of the metal materials, followed by cleaving at the (110) plane of the wafer, resulting in a semiconductor laser array device 9.
The optical field distribution of beams oscillated by the laser array device 9 and the far-field pattern attained by the laser array device 9 are shown in FIGS. 8 and 9, respectively, indicating that the optical phase shift between the adjacent waveguides is 180.degree..
The reason why the conventional semiconductor laser array device 9 having a plurality of waveguides is operated in a 180.degree. phase-shift mode is that laser light is absorbed by the optical coupling area between the adjacent waveguides, which makes threshold gain of the 180.degree. phase-shift mode significantly low.
The above-mentioned phenomenon that the conventional laser array device 9 is operated in a 180.degree. phase-shift mode can be also explained by reference to FIG. 10, which shows the independence threshold gain of all allowed array modes (.nu.=1, 2 and 3) of a triple lasing filament array 9 on the difference in refractive index in the lateral direction. This independence is obtained by an analysis of the waveguides. It can be also seen from FIG. 10 that the conventional laser array device 9 selectively and stably oscillates laser light in a 180.degree. phase-shift mode. As mentioned above, such a 180.degree. phase-shift mode attains a far-field pattern having two peaks, which causes difficulty in the concentration of laser light into a spot fashion by means of any optical lens.
Moreover, the conventional laser array device 9 oscillates laser light in array modes other than the 0.degree. phase-shift mode and the 180.degree. phase-shift mode, thereby producing output light with a plurality of beams. In addition, two or more array modes are superposed without interference therebetween, thereby producing output light with thick beams.
The above-mentioned phenomena are unfavorable in the use of semiconductor laser array devices. Semiconductor laser array devices, which attain a single oscillation mode and oscillate output power light of a single beam, are required.